1. Field of the Invention
The present disclosure relates to a semiconductor device.
2. Description of the Related Art
A lithium-ion battery includes a charge and discharge protection device within a battery pack for preventing over-charge and over-discharge of a battery cell, as well as the battery cell provided within the battery pack in order to protect the battery from over-charge and over-discharge. This charge and discharge protection device includes a bidirectional switch, and thus needs ON-OFF control for each of bidirectional currents. Accordingly, the charge and discharge protection device is generally constituted by two power MOSFETs (Metal Oxide Semiconductor Field Effect Transistors).
According to this type of charge and discharge protection device equipped with two power MOSFETs, under a first condition, a positive voltage with respect to a source of a first MOSFET is applied to a gate of the first MOSFET to produce an ON-state between a drain and the source of the first MOSFET and allow a function of a diode by a second MOSFET. As a result, a current flows from a source of the second MOSFET to the source of the first MOSFET. Under a second condition, a positive voltage with respect to the source of the second MOSFET is applied to a gate of the second MOSFET to produce an ON-state between a drain and the source of the second MOSFET and allow a function of a diode by the first MOSFET. As a result, a current flows from the source of the first MOSFET to the source of the second MOSFET. Bidirectional currents are controllable based on the foregoing operations. However, the structure equipped with two power MOSFETs as noted above is difficult to meet a demand for size reduction of a cellular phone or other devices.
For example, a semiconductor device disclosed in Unexamined Japanese Patent Publication No. 2008-53378 developed to overcome this problem includes a source area formed throughout a surface of an operation area, a first back gate area formed between trenches and below the source area, a second back gate area formed out of the source area and connected with the first back gate area, a first electrode layer contacting the source area and formed throughout the surface of the operation area, and a second electrode layer contacting the second back gate area and formed on an outer periphery of the first electrode layer. This structure is capable of applying a voltage separately to the first electrode layer and the second electrode layer, and therefore needs only one power MOSFET for bidirectional current control.
Unexamined Japanese Patent Publication No. 2008-53378 adopts ion implantation as a method for forming the first back gate electrode, while Unexamined Japanese Patent Publication No. 2002-270841 discloses a method using doped polysilicon for formation of the electrode.